A comprehensive study to understand the mechanism of action of growth hormone on macromolecular biosynthetic and energy transducing processes of rat liver mitochondria is proposed. Growth hormone was found to significantly affect rat liver mitochondrial protein synthesis, cytochrome and flavin levels, and activities of cytochrome and NADH oxidases. Estradiol and cortisol inhibited the hormone effect on protein synthesis. The hormone also affected mitochondrial morphology and the half-life for the turnover of mitochondrial proteins. Detailed kinetics of binding of fluorescent dye probes like 8-anilino-l-naphthalene sulfonate, 6-p-toluidino-2-naphthaene sulfonate and pyrene-3-sulfonate to whole mitochondria and electron transport particles in unenergized, energized and uncoupled states will be measured. Conformational transitions as detected by these probes during electron transport will be evaluated. Mitochondrial gene products will be evaluated, on the one hand, with the concentration of cytochromes and other respiratory components, and on the other, with the parameters of structural or conformational transitions. Any apparent relationship among these mitochondrial processes would aid in our understanding of the biogenesis of mitochondria in eucaryotic cells. The effect of age at hypophysectomy on the influence of growth hormone on some of these processes will also be investigated.